c: Try the type with the previous function attributes

c: Try the type with the previous function attributes

When there are 2 conflicting function declarations, try the new type
with the previous TYPE_ATTRIBUTES if the current declaration has no
TYPE_ATTRIBUTES to support

extern void foo (void) __attribute__((nocf_check));

void
foo (void)
{
}

instead of issuing an error:

$ gcc -O2 -fcf-protection -S x.c
x.c:4:1: error: conflicting types for ‘foo’; have ‘void(void)’
    4 | foo (void)
      | ^~~
x.c:1:13: note: previous declaration of ‘foo’ with type ‘void(void)’
    1 | extern void foo (void) __attribute__((nocf_check));
      |             ^~~

The resulting function definition is compatible with the previous
declaration.

gcc/c/

	PR c/122427
	* c-decl.cc (diagnose_mismatched_decls): For FUNCTION_DECL, if
	OLDDECL has TYPE_ATTRIBUTES and NEWDECL doesn't, try the type
	with the OLDDECL attributes.

gcc/testsuite/

	PR c/122427
	* g++.target/i386/cf_check-1.C: New test.
	* g++.target/i386/cf_check-2.C: Likewise.
	* g++.target/i386/cf_check-3.C: Likewise.
	* g++.target/i386/cf_check-4.C: Likewise.
	* gcc.target/i386/cf_check-7.c: Likewise.
	* gcc.target/i386/cf_check-8.c: Likewise.
	* gcc.target/i386/cf_check-9.c: Likewise.
	* gcc.target/i386/cf_check-10.c: Likewise.
	* gcc.target/i386/cf_check-11.c: Likewise.
	* gcc.target/i386/no-callee-saved-12.c: Remove dg-error.
	* gcc.target/i386/preserve-none-17.c: Likewise.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>